How to scale an image source at a specific resolution
You can set the Resolution Size (in pixels) to the desired output size in the Scene Dimensions section of the properties window..
Using a Scale Node,  will rescale an image connected to it to the predetermined output resolution.

If you want to double the number of pixels just render at 200% (in this example rendering a 512 x 512 pixel image will result in an image of 1024 x 1024 pixels). To cut the number of pixels in half, render at 50%.

Note that if the images are a different aspect ratio than the output, you have three choices:
stretch: deform the image to match within the aspect ratio of the container)
Fit: keep the images proportions but fill the remaining either with a transparent background or with black (if the &quot;Use Alpha&quot; option is unchecked)
or Crop Not stretching the image, resizing to fit vertically center cropping horizontally.